This invention relates generally to trains and train wheels and more particularly to a novel tire profile that improves train stability and energy efficiency.
Train tire profile includes several sections. A flange section protrudes downward from the side of the train wheel and extends over the lateral side of a train track. A fillet extends upward along a field side of the flange providing transition to a straight conical wheel tread section. The wheel tread section serves as the major load bearing surface that supports the train wheels on a train track. The art uses tread profile of two opposing tires each on one of two rails to steer. Two opposing tires are a wheelset. The flange provides steering when rail curve exceeds capability of treads to steer without flange contact.
Two main factors must be considered when designing tire profiles for use with railed devices. The first is the dynamic stability of the vehicle at various speeds throughout its operating speed range. When in transit, a train experiences lateral oscillations known as "hunting". Wheel hunting results in the wheels oscillating laterally back and forth between the wheel flanges. The maximum speed or critical speed of the train is determined by the onset of unstable, undesirable wheelset hunting. For example, if the train goes too fast, the force of the lateral oscillations will overcome the flange barrier and cause the train to derail. Hunting is caused by the dynamics between the wheel tread profile and the rail. Increasing the slope of the wheel tread too fast toward flange increases forces causing hunting and, therefore, lowers the critical speed of the vehicle. Decreasing slope of wheel tread toward flange decreases steering forces, also lowering the critical hunting speed. This is the measure of mismatch. These limits define critical mismatch threshold.
A second factor involved with train stability is the ability of the vehicle to negotiate track curves. This curving ability is determined primarily by the ability of the opposing tires of wheelsets to follow the track curves. Optimally, the wheelsets should perform a purely rolling motion in the track curves without any contact between the wheel flanges and the rails. This requires steering forces to be generated by the sloped wheel tread independently of the wheel flange permitting the wheelset to yaw or rotate about a vertical axis which may be through its center. Oscillation of steering forces happen around vertical axis through its center of gravity (mass). This oscillation is a metric space. The oscillation of wheelset results in hunting. The steering forces move the train wheelsets into a more radial position (axle 82 FIG. 17) with respect to the track curves, thus, increasing train stability around curves.
A wheelset includes two opposite wheels that may be joined together by an axle. With a conical (straight taper) wheel tread the conicity remains virtually constant with lateral deflection of a wheelset relative to the track. That is, straight taper wheel treads have a constant slope. In other words, the conicity of each wheel remains the same irrespective of whether the wheelset runs centrally on the track or is deflected closer to one rail. Increasing the conicity of the wheel tread improves the steering ability of the wheelsets because of the increased steering force. However, increased conicity also increases the oscillation of the wheelset. Oscillation of wheelset results in hunting. Therefore, with regard to the conicity of wheel treads, there is a conflict between the requirement for hunting stability and increased vehicle speed and for good curving ability of the wheelsets.
U.S. Pat. No. 4,294,482 to Scheffel et al., discusses a profiled wheel tread that is made up of a combination of discrete circle and line segments. In the art the term "profiled" is used in relation to wheels having a curved tread section and distinguishes such wheels from conventional wheels having a straight conical (linear sloped) tread section. The profiled tread in Scheffel et. al., utilizes multiple discrete curve sections each having a separate radius that are combined to form a non-continuous curve. The term conicity is imprecise. The curve radii of the profile increase from taping line to the flange. This is thought to reduce conicity of the profile. It reduces conicity of profile when compared to profile of constant radius between taping line and fillet. Thus, the tire tread conicity has essentially a "droop" characteristic with increasing lateral deflection toward the flange compared to tread of constant radius. A train profile with multiple curve discontinuities and a "drooping" characteristic will initiate vibration between the wheelset and the train. Because the wheel tread also has relatively high conicity of tread slope change at the taping line, the tire is also more likely to hunt in relation to a straight taper. The minimum radius is at the taping line with larger radii toward fillet.
U.S. Pat. No. 5,295,624 to Ziethen et al. discusses means for varying the train rail profile to extend wheel wear and track durability. Ziethen et al., however, does not suggest means for reducing vibration and hunting in train treads. The definition of concern is that radius of tire tread is larger than radius of contacted rail. Also definition of concern is that there would not be two point contact between tire profile and rail. In FIG. 1 of Ziethen the lower continuous line (10) looks like it goes horizontal at coordinate origin [line 57 col 3] thus indicating a discrete radius of tread in accordance with that invention to the extent shown in their figure. This meets their requirement that tread radius is larger than contacted rail radius. Therefore, in a manner similar to Scheffel, FIG. 1 in Ziethen suggests using discrete change of tread section. Therefore, the Ziethen tread profile begins with a linear wheel tread and then abruptly changes to a radius in tread toward the fillet. The abrupt change in curvature in the tread section induces vibration and increases wheel resistance when the wheelset is laterally displaced on standard rail. This is not an absorptive barrier. Thus, the profile in Ziethen is not energy efficient and further does not provide additional steering forces different than prior art.
U.S. Pat. No. 1,298,628 to Coda describes a wheel profile including two separate sections that intersect at the taping line. The profile is a wheel tread having a conical (linear) outer portion tangent to a portion of continually increasing conicity to the wheel flange. U.S. Pat. No. 1,783,705 to Emerson shows a substantially linear tread profile that is profiled to match the curvature of a corresponding inner rounded corner of railhead. U.S. Pat. No. 994,350 to Vial, discusses a wheel having both a concave throat section and a separate convex tread section toward the fillet. Coda, Emerson and Vial, similar to the other patents previously listed, have either straight conical tread profiles or discontinuous tread that include a combination of different sections each having a different curvature formula. Tread profiles with elliptical or circular profiles include a harmonic frequency that causes periodic oscillation under certain track conditions. Thus, the problems of vibration, limited steering forces and hunting still exist.
Another problem with train tires is excessive wear both at the tread and flange. For example, linear tread profiles typically exhibit excessive wear next to the flange. Contrary to normal expectations, an increase in the contact area between the wheel and the rail does not necessarily decrease tread wear. Research has now shown that the rate of wear of the tread, in fact, increases when the shape of the tread approaches the shape of the rail head. This is because the wheel-tread/rail-head contact area increases to such an extent that there is a disadvantageous increase of the creep forces or slip of the wheel on the rail. The increased creep forces significantly increase the wear rate offsetting reduction in wear rate due to increased contact area. Thus, it is predicted that tread profiles that match the railhead, such as Ziethen, have a tendency to wear faster according to Scheffel.
Many standard train tire treads have linear profiles but, due to wear, rapidly become nonlinear. This small amount of nonlinear wear causes hunting as described by John F. Leary in paper entitled America Adopts Worn Wheel Profiles in Railway Gazette International, July 1990. Additional information is provided by John F. Leary, Stephen N. Handle and Britto Rajkumar in a paper entitled: "Development of Freight Car Wheel Profiles--A Case Study", in Wear, 144 (1991) pages 353-362, Association of American Railroads, Pueblo, Colo. The, nonlinear circle segments used for defining the tire profiles in both Scheffel and Ziethen also suggest profiling the tire tread into nonlinear shapes. Thus, present nonlinear profiled tire treads have increased steering forces that induce hunting.
Accordingly, a need remains for a train tire profile that is both resistant to hunting and vibration, provides steering forces during turns and is energy efficient.